Proteins have been generally subjected to size separation on the basis of SDS-PAGE (polyacrylamide gel electrophoresis) method, and detected. One method of applying this technique to capillary electrophoresis is SDS-CGE (capillary gel electrophoresis), and a method of further applying this method to microchip electrophoresis includes analysis with Agilent 2100 Bioanalyzer (manufactured by Agilent Technologies) using Protein 200 Kit. The proteins have various electric charges in native states, and the positively charged proteins are considered not to migrate in a positive electric field. Therefore, in the above-mentioned conventional method, it is usually necessary to carry out a heat-denaturing treatment of a protein as a pretreatment so that all of the test proteins have negative charges. The heat-denaturing treatment is usually accomplished by heating a protein at 95° to 100° C. for several minutes in a surfactant sodium dodecylsulfate (SDS) solution and a reducing agent such as 2-mercaptoethanol or dithiothreitol. 2-Mercaptoethanol or dithiothreitol is used for cleaving S—S bond, and SDS is used for making the electric charges in the entire proteins negative.
Therefore, in the conventional method such as SDS-PAGE method or SDS-CGE method, there is a defect that the time period for this pretreatment step is not shortened, and the procedure is complicated, even though the analyzing step is made rapid by the microchip electrophoresis. Therefore, there has been desired an even more rapid electrophoresis method of a protein. Furthermore, when a biological sample is analyzed, it is necessary to analyze a very small amount of a protein in the sample, so that there has been desired electrophoresis method having even higher sensitivity.